Glutamate transmission constitutes a major portion of information transfer between neurons in the central nervous system. It is tightly regulated to occur as a point-to-point process across synapses, but it may also escape the synapse to activate extrasynaptic receptors or receptors in neighboring synapses. The regulation and contribution of such glutamate spillover has yet to be fully understood. On one hand, it may represent an important extra layer of signaling in the course of regular transmission. On the other, excess extrasynaptic signaling is a component of many neurodegenerative conditions including Alzheimer's disease, amyotrophic lateral sclerosis, multiple sclerosis, and glial-derived brain tumors. It has recently been shown that spillover transmission in the cerebellum mediates communication between excitatory climbing fibers (CFs) and molecular layer interneurons (MLIs), and that this form of signaling occurs in the absence of anatomically-defined synaptic structures. It thus affords the opportunity to study physiologically relevant glutamate spillover i isolation from conventional synaptic transmission. We have found that glutamate release from single CFs spills over to multiple interneurons and functionally segregates these cells based on their location relative to neurotransmitter release. Furthermore, CF-evoked glutamate release onto a single Purkinje cell generates biphasic patterns of inhibition and disinhibition to neighboring Purkinje cells resulting from the convergence of activity from excited and inhibited MLIs. To further explore the mechanisms and consequences of the CF-MLI spillover connection, we will 1) define the functional connectivity that underlies spillover-mediated inhibition and 2) determine the subcellular pattern of spillover activation of postsynaptic receptors. These goals will be achieved using a variety of techniques including electrophysiology and Ca2+-imaging under the expert guidance of the sponsor and collaborators. The proposed work will provide a detailed understanding of the effect of CF spillover on local circuit processing in the cerebellar cortex. This will in turn benefit our understanding of many disease states that involve altered cerebellar processing, including spinocerebellar ataxia, schizophrenia, and autism spectrum disorders. In addition these studies will help to distinguish between the mechanisms of appropriate and inappropriate involvement of extrasynaptic glutamate signaling, possibly providing targets for its modulation as therapy in the diseased states discussed above. The training plan for the PI includes participation in yearly seminar speaking opportunities, formal lab meetings, journal clubs, presentations at national meetings, formal and informal training in ethical scientific practices, and regular meetings with the mentor.